5.32.5.1 General principles

When an MA PDU Session is established, the network may provide the UE with Measurement Assistance Information. This information assists the UE in determining which measurements shall be performed over both accesses, as well as whether measurement reports need to be sent to the network.

Measurement Assistance Information shall include the addressing information of a Performance Measurement Function (PMF) in the UPF, the UE can send PMF protocol messages to:

• For a PDU Session of IP type, Measurement Assistance Information contains one IP address for the PMF, one UDP port associated with 3GPP access and another UDP port associated with non-3GPP access. PMF messages sent by UE to one of these UDP ports, shall be transmitted to UPF via the QoS Flow associated with the default QoS rule.
• For a PDU Session of Ethernet type, Measurement Assistance Information contains one MAC address associated with 3GPP access and another MAC address associated with non-3GPP access. PMF messages sent by UE to one of these MAC addresses, shall be transmitted to UPF via the QoS Flow associated with the default QoS rule.

NOTE 1: To protect the PMF in the UPF (e.g. to block DDOS to the PMF), the IP addresses of the PMF are only accessible from the UE IP address via the N3/N9 interface.

NOTE 2: After the MA PDU Session is released, the same UE IP address/prefix is not allocated to another UE for MA PDU Session in a short time.

If the SMF determines that access performance measurements per QoS Flow shall be applied for the MA PDU Session, then the Measurement Assistance Information shall also include a list of QoS Flows on which access performance measurements may be performed. For each QoS Flow in this list, the following information is included:

• The QFI of the associated QoS Flow.
• For a PDU Session of IP type, one UDP port associated with 3GPP access and another UDP port associated with non-3GPP access. PMF messages sent by UE to one of these UDP ports, shall be transmitted to UPF via the associated QoS Flow.
• For a PDU Session of Ethernet type, one MAC address associated with 3GPP access and another MAC address associated with non-3GPP access. PMF messages sent by UE to one of these MAC addresses, shall be transmitted to UPF via the associated QoS Flow.

The QoS rules and the N4 rules provided by SMF to UE and to UPF respectively shall include information (e.g. packet filters containing the UDP port or the MAC address associated with a QoS Flow), which enables the UE and UPF to route a PMF message to a specific QoS Flow.

The UE and the UPF may need to perform access performance measurements in order to estimate the Round-Trip Time (RTT) and/or the Packet Loss Rate (PLR) that an SDF is expected to experience when transmitted on a certain access type. Based on these measurements and the provisioned ATSSS rules in the UE and MAR rules in the UPF, the UE and the UPF decide how to distribute the traffic of an SDF across the two accesses.

If the UE and the UPF decide to initiate access performance measurements to estimate the RTT and/or the PLR for an SDF, the access performance measurements shall be performed either

(a) using the QoS Flow associated with the default QoS rule; or

(b) using the target QoS Flow, which is the QoS Flow that the SDF traffic is transmitted on.

When the access performance measurements are using the target QoS Flow, it is termed that "access performance measurements per QoS Flow" are applied for the MA PDU Session.

The UE shall indicate in its ATSSS capabilities that it supports access performance measurements per QoS Flow. Based on this UE capability and other information (such as local policy), the SMF determines whether access performance measurements per QoS Flow shall be applied for the MA PDU Session or not. If the SMF determines that access performance measurements per QoS Flow shall be applied for the MA PDU Session, then:

• The SMF determines a list of QoS Flows over which access performance measurements may be performed and provides this list to the UE (within the Measurement Assistance Information) and to the UPF.
• The UE and the UPF may initiate access performance measurements on one or more of the QoS Flows included in this list. The UE and the UPF shall be able to receive and respond to PMF messages sent on any QoS Flow included in this list.
• The SMF may update the list of QoS Flows over which access performance measurements may be performed during the lifetime of a MA PDU Session, e.g. when a new PCC rule that could benefit from PMF access performance measurements is bound to a QoS Flow.

NOTE 3: The SMF can e.g. add a QoS Flow into the list when at least one PCC Rule is bound to that QoS Flow that is using one of the Steering Modes where performance measurements via PMF are applicable, such as Lowest Delay Steering Mode or a Steering Mode where threshold values have been provided.

The UE shall perform access performance measurements per QoS Flow only when this is explicitly indicated in the Measurement Assistance Information, i.e. only when the UE receives the list of QoS Flows over which access performance measurements may be performed. Otherwise, the UE shall perform access performance measurements based on the QoS Flow associated with the default QoS rule. The UPF shall perform access performance measurements per QoS Flow only when this is explicitly indicated by SMF, i.e. only when the UPF receives the list of QoS Flows over which access performance measurements may be performed. Otherwise, the UPF shall perform access performance measurements based on the QoS Flow associated with the default QoS rule. In this case the UPF learns what QoS Flow to use as described in TS 29.244 [65].

The UE and the UPF may decide not to initiate access performance measurements using PMF over a certain target QoS Flow, when they already have access performance measurements for another target QoS Flow which they determine can be reused.

NOTE 4: How the UE and UPF determine that the performance measurements using a certain target QoS Flow apply to another target QoS Flow is based on implementation, e.g. AN resource to QoS Flow mapping in the UE or getting similar access measurements results with other QoS Flow.

When access performance measurements for an SDF are performed based on the target QoS Flow, the UE needs to be able to determine the QoS Flow a downlink packet arrives on. In order to enable this, the SMF shall include downlink Packet Filter information in the QoS rule provided to UE matching this SDF, unless Reflective QoS is used for the SDF.

NOTE 5: For example, if a QoS Flow requires to activate Reflective QoS, the SMF does not need to provide downlink QoS Flow information for the QoS Flow to minimize usage of packet filters. When a data packet is received over a QoS Flow, the UE can decide whether to check the downlink QoS Flow information based on the existence of SDAP header for the QoS Flow.

The addressing information of the PMF in the UPF is retrieved by the SMF from the UPF during N4 session establishment. If the UPF receives from the SMF, during N4 session establishment or modification procedure, a list of QoS Flows over which access performance measurements may be performed, the UPF allocates different UDP ports per QoS Flow per access for IP PDU sessions, or allocates different MAC addresses per QoS Flow per access for Ethernet PDU sessions. For IP PDU sessions, the UPF sends the PMF IP addressing information and the UDP ports with the QFI of the associated QoS Flow to the SMF. For Ethernet PDU sessions, the UPF sends the MAC addresses with the QFI of the associated QoS Flow to the SMF.

The following PMF protocol messages can be exchanged between the UE and the UPF:

• Messages to allow for Round Trip Time (RTT) measurements, i.e. when the "Smallest Delay" steering mode is used or when either "Priority-based", "Load-Balancing" or "Redundant" steering mode is used with RTT threshold value being applied;
• Messages to allow for Packet Loss Rate (PLR) measurements, i.e. when steering mode is used either "Priority-based", "Load-Balancing" or "Redundant" steering mode is used with PLR threshold value being applied;
• Messages for reporting Access availability/unavailability by the UE to the UPF.
• Messages for sending UE-assistance data to UPF. Such messages may be sent from the UE to UPF only when the UE receives the UE-assistance indicator in an ATSSS rule, as specified in clause 5.32.8. Further details are provided in clause 5.32.5.5.
• Messages for sending Suspend Traffic Duplication and Resume Traffic Duplication from UPF to UE to suspend or resume traffic duplication as defined in clause 5.32.5.6.

Since steering modes can be different in up-link and down-link, the UE needs to be able to handle PMF protocol messages for RTT and PLR measurements received from UPF even if it is not using one of the steering modes associated with the RTT and PLR measurements (and vice versa).

The PMF protocol is specified in TS 24.193 [109].

The PMF protocol messages used for access availability/unavailability reports shall be sent on the QoS Flow associated with the default QoS rule. The PMF protocol messages used for access performance measurements shall be sent either on the QoS Flow associated with the default QoS rule, or on the target QoS Flow, as specified above.

The QoS Flow associated with the default QoS rule for MA PDU Session is Non-GBR QoS Flow.

The UE shall not apply the ATSSS rules and the UPF shall not apply the MAR rules for the PMF protocol messages.

When the UE requests a MA PDU session and indicates it is capable to support one or more of the steering functionalities below:

• the "MPTCP functionality with any steering mode supported" capability and the "ATSSS-LL functionality with only the Active-Standby steering mode supported" capability (as specified in clause 5.32.6.1);
• the "MPQUIC-UDP functionality with any steering mode supported" capability and the ATSSS-LL functionality with only the Active-Standby steering mode supported" capability (as specified in clause 5.32.6.1);
• the "MPQUIC-IP functionality with any steering mode supported" capability and the ATSSS-LL functionality with only the Active-Standby steering mode supported" capability;
• the "MPQUIC-E functionality with any steering mode supported" capability and the "ATSSS-LL functionality with only the Active-Standby steering mode supported" capability;

the network may send Measurement Assistance Information for the UE to send Access availability/unavailability reports to the UPF. In this case, the UE and UPF shall not perform RTT and PLR measurements using PMF as the UE and UPF can use measurements available at the MPTCP layer and/or at the MPQUIC layer.